Many of today's sophisticated communication radio receivers are extremely sensitive and need to demodulate signals that are well below −100 dBm, that is, less than 100 fW. While this low signal threshold increases the range between the transmitter and the receiver, it also makes these receivers highly susceptible to destruction by high energy electromagnetic pulses. These high power transients can be caused on purpose, or could be accidental (e.g. from having the receiver cross the path of a high power radar beam) or could be a natural event (e.g., a lightning strike).
The adverse effect of high energy electromagnetic radiation on communication radios has been known for a long time. The usual protection against such high levels of EM radiation has either been fuses or component circuit breakers. Fuses, once activated, need to be replaced before the radio can operate again. Component circuit breakers such as semiconductor diodes or capacitive shunts are limited in the amount of current/voltage that can be shunted. Although it is possible to place most of the electronic components of the receiver front end inside EM shielded enclosures, prior receiver front ends still need to be electrically connected to the antenna. This electrical connection provides a path for the high-energy radiation to penetrate the enclosure.
An electrically insulating optical connection between an antenna and receiver electronics is described in an article by R. C. J. Hsu, A. Ayazi, B. Houshmand, and B. Jalali, “All-dielectric photonic-assisted radio front-end technology”, Nature Photonics, vol. 1, September 2007, pp. 535-538). This prior art apparatus uses an electro-optic modulator to sense the level of the microwave signal coupled into a dielectric resonator antenna. The electro-optic modulator is part of an RF-photonic link that provides electrical isolation between the antenna and the rest of the microwave receiver front-end (which contains the sensitive radio components), thereby achieving tolerance to unwanted high power electromagnetic radiation. This prior approach is based on an electro-optic microdisk resonator (acting as the modulator) that is intimately in contact with the dielectric resonator antenna. Because of its resonant antenna, this prior art apparatus couples in substantial power from incident radiation at only those frequencies that match the resonant frequencies of the antenna. Nevertheless, despite its all dielectric construction, the power at those resonant frequencies is slowly absorbed by the dielectric antenna of this prior art apparatus over a characteristic time that is inversely proportional to the resonator linewidth. If that absorbed power is sufficiently high, it can damage or alter the antenna and the modulator.
Resonators (used for both the antenna and the modulator of this prior art apparatus) have very narrow bandwidth and can be operated only at particular frequencies. In contrast, the presently disclosed front-end sensor head makes use of a wideband, non-resonator antenna to capture and concentrate the incident radiation. It also uses optical-waveguide based electro-optic modulators that are built into a wide band transverse electromagnetic (TEM) waveguide. Such optoelectronic modulators, whether they involve distributed Bragg-reflector gratings or phase modulators in interferometric configurations, have much wider bandwidth than the microdisk resonators of the prior-art apparatus. The bandwidth of the presently disclosed front end can be greater than 5-10%, much larger than the bandwidth of less than 0.1% achieved with the prior art resonator-based approach.
The present disclosure involves an EM-field sensitive structure that does not need to absorb any incident EM energy. Instead, the incident EM radiation passes through the sensor head and exits the sensor head. Since the sensor assembly is electrically passive, very little RF power is actually absorbed, provided that low loss dielectric materials are used and provided the electric fields in the structure remain below the dielectric breakdown of the constituent filling materials. Thus, there is no need for fuses or component circuit breakers. In addition, since the only connection to the EM shielded enclosure of the front end comprises dielectric optical fibers of very small diameter, there is almost no path for penetration of any incident high-energy radiation into that shielded enclosure.
Some embodiments of the present disclosure involve an array of sensor-head elements, with each of those elements comprising an antenna, such as a TEM horn, coupled to a TEM waveguide that contains at least one electro-optic modulator. The prior art also includes U.S. Pat. No. 6,703,596 (entitled “Apparatus and System for Imaging Radio Frequency Electromagnetic Signals,” invented by Joseph E. Moran) which teaches an apparatus and system for imaging radio frequency electromagnetic signals. In this prior antenna array, each antenna element in the antenna array is connected to a separate electro-optic modulator and also serves as the electrodes of the electro-optic modulator. These antenna elements do not act as microwave waveguides nor are they coupled to microwave waveguides that supply a modulating RF signal to the electro-optic modulator. In contrast, for the presently disclosed front end array, the electro-optic modulator in each array element is built into a TEM microwave waveguide that carries the modulating RF signal to electro-optic modulator located in a slice of that microwave waveguide. Thus, the presently disclosed integrated structure comprising TEM microwave (or RF) waveguide and electro-optic modulator is better able to withstand the high electric fields and high currents resulting from an incident high-energy EM pulse.
Some embodiments of the present disclosure involve an array of sensor-head elements containing multiple electro-optic modulators that are optically connected in series to increase the overall depth of modulation. A prior art electro-optic modulator that has multiple series-connected sections is described in U.S. Pat. No. 5,076,622 (entitled “Antenna-Fed Electro-optic Modulator,” invented by William B. Bridges). This prior patent describes an RF waveguide that couples an EM field onto an array of printed-circuit electrodes. The printed-circuit electrodes, with one set of electrodes for each section of electro-optic modulator, act as a phased array of antenna elements that receive the EM field of the TEM RF waveguide. Those printed-circuit electrodes constrain the electric field that modulates the light propagating in the modulator. In contrast, for the present disclosure, the TEM RF waveguide itself constrains the electric field that modulates the light propagating in the modulator—there is no need for additional printed circuit electrodes. Thus, the presently disclosed integrated structure is better able to withstand the high electric fields and high currents resulting from an incident high-energy EM pulse.
In this prior art apparatus, the relative phase delay of the RF signals received by the array of electrodes is matched to the time delay of the light propagating in the series connection of electro-optic modulator sections. The desired relative phase delay is achieved by properly placing and aligning the array of electrodes within the RF waveguide such that EM field of the waveguide couples into those electrodes at successively delayed instances of time. In contrast to this prior art, the presently disclosed array achieves the desired phase delay matching by separately delaying the RF signals for driving different modulators of the array by means of separate TEM waveguides (into which the modulators are integrated) that have differing microwave propagation velocities or electrical lengths.